What's the best way to go about upgrading? Do 73 to 76 control arms just bolt in? There's a few parts cars around and if that's the best way I'll go buy control arms off them. Or is it better to buy one of the kits.
So the partial answer to the OP's question is, the UCA's can be useful off the parts car you found. You could use the LBJ, but only if you are going really low buck or they are in really good shape.
You can use your current LCA, only difference between any of them is the sway bar mount if they have one.
My list would be any '73+ UCA and LBJ, disk spindles and the associated brake stuff. Should be all you need.
Dr Diff sells a kit for $720 and includes the spindle and brakes, adapters for the small UBJ and new LBJs.
https://www.doctordiff.com/reproduction-disc-brake-knuckles.html
I get used UCA and new spindles myself, but I am a cheap SOB. You could end up spending some sweat equity to get the UCA though, not unusual that the bolts rust to the bushings and force drastic measures to get them out of the car. Plus the work of replacing the UBJ if they are worn.